AL Paper chromatography SQ

用戶31980

2013-03-22 4:52 am

07 AL IA 3(b)
A mixture of Fe3+(aq) and Cu2+(aq) is separated by paper chromatography using a mixture of propanone and 6M HCl(aq) as the mobile phase. Suggest how you would identify chemically the Fe3+(aq) and Cu2+(aq) on the chromatographic paper.

As the two ions have different colours, I thought they can be simply distinguished by their colours, i.e. a yellow brown spot will appear, representing the presence of Fe3+(aq) and a blue spot will appear, representing the presence of Cu2+(aq)

but in the marking scheme, it said
Place the chromatographic paper in an atmosphere of ammonia.
Fe3+(aq) reacts with OH-(aq) to give brown Fe(OH)3(s)
Cu2+(aq) reacts with NH3(aq) to give deep blue complex [Cu(NH3)4]2+ (aq)

why?

can the chromatographic paper in an atmosphere of sodium hydroxide solution instead? why?

plz help me:)

回答 (1)

[匿名]

2013-03-22 8:38 pm

✔ 最佳答案

In the marking scheme, it said
Place the chromatographic paper in an atmosphere of ammonia.
Fe³⁺ (aq) reacts with OH⁻(aq) to give brown Fe(OH)3(s)
Cu²⁺(aq) reacts with NH3(aq)to give deep blue complex [Cu(NH3)4]²⁺(aq)
why?

This is because both the yellow Fe³⁺ (aq) and the pale blue Cu²⁺(aq) are pale incolour. When each of them is in the formof a small spot in the chromatographic paper, the colour is too pale to beobserved. Therefore, they must convertedto substances in deeper colours, i.e. by using ammonia, Fe³⁺ (aq) is converted to brownFe(OH)3(s), and Cu²⁺(aq) is converted to deepblue [Cu(NH3)4]²⁺(aq).

=====
Can the chromatographic paper in an atmosphere of sodium hydroxide solutioninstead? why?

No.
Atmosphere means it is in gaseous state. Sodium hydroxide solution is in liquid form, and thus their cannot be anatmosphere of sodium hydroxide solution.
If sodium hydroxide solution is used instead of ammonia gas, both spots willdissolve in the sodium hydroxide solution.

參考: Adam

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